Summary of Securities in Unrealized Loss Position and Included in Accumulated Other Comprehensive Income, Aggregated by Length of Time Investments (Detail) (USD $) In Thousands, unless otherwise specified	Feb. 28, 2014	May 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Total investments with unrealized losses, fair value	$ 38,078	$ 36,582
Unrealized losses with a loss position for less than 12 months, fair value	23,025	36,327
Unrealized losses with a loss position for more than 12 months, fair value	15,053	255
Total investments with unrealized losses, gross unrealized losses	(1,768)	(1,034)
Unrealized losses with a loss position for less than 12 months, gross unrealized losses	(1,071)	(956)
Unrealized losses with a loss position for more than 12 months, gross unrealized losses	$ (697)	$ (78)